1. Field of the Invention
The present invention relates to an apparatus and method for detecting a laundry weight in a washing machine employing a sensorless BLDC motor, and more particularly, to an apparatus and method for detecting a laundry weight in a washing machine employing a sensorless BLDC motor that is capable of detecting a laundry weight in a dry state by using a motor speed difference according to a laundry weight.
2. Description of the Background Art
Recently, in order to improve a washing degree of the laundry and reduce a noise and an energy of a washing machine, an inverter unit which is able to variably control freely a motor speed has been adopted to a washing machine, and in addition to this, in order to adjust a water level and a detergent amount, a method for detecting a laundry weight (a weight of the laundry) in a washing machine has been adopted.
FIG. 1 is a circuit diagram showing a driving device of a BLDC motor in accordance with a conventional art.
As shown in FIG. 1, the driving device includes a rectifying unit 11 for rectifying and smoothing an AC power, an inverter unit 12 for applying the rectified and smoothed voltage and current to a BLDC motor 13 and controlling the speed of a rotor (not shown) of the motor; a position detector 14 for detecting a position of a rotor of the motor 13 by using a hall unit 17 attached at the motor 13; a microcomputer 15 for receiving a position detect signal from the position detector 14 and controlling the speed of the motor 13; and a pulse width modulator 16 for receiving a control signal from the microcomputer 15, modulating an output pulse width and controlling the inverter unit 12.
The conventional driving device of the BLDC motor constructed as described above will now be explained.
When the rectifier 11 rectifies and smoothes an input AC power and applies it to the inverter unit 12, the inverter unit 12 receives a drive signal from the pulse width modulator 16 and applies a three-phase voltage to the motor 13.
Accordingly, the motor 13 starts rotating. At this time, the position detector 14 detects a position of the rotor of the motor 13 by using the hall unit 17 attached at the motor 13, and inputs a voltage value according to the detected position value to the microcomputer 15.
The microcomputer 15 computes the speed of the motor 13 with the voltage value, compares it with a pre-set reference speed, and outputs a control signal for rotating the motor 13 at a desired speed to the pulse width modulator 16.
Upon receipt of the control signal from the microcomputer 15, the pulse width modulator 16 converts a level of the control signal and outputs it to the inverter unit 12.
There are two methods for detecting a laundry weight by using the BLDC motor.
The first one is a method that the motor 13 is accelerated to a certain speed for a predetermined time period and then a power is cut off so as to check a rate at which the speed of the motor 13 is decelerated, to thereby measure a laundry weight. That is, in this method, the inertia of the motor 13 according to the laundry weight is measured to detect the laundry weight. This method is mostly adopted to a washing machine in which a pulsator (not shown) reduces a rotational force of the motor 13 or changes a direction of the motor by a clutch and a gear.
The second one is a method that time during which the motor 13 reaches a certain speed is measured. In this method, since the motor 13 is directly connected to the pulsator (not shown) or an inner tub (not shown) and driven, this method is useful for a washing machine adopting a direct driving method.
The laundry weight detecting method adopted to a washing machine of the direct driving method will now be described with reference to FIGS. 2A through 2E.
After the motor 13 is aligned at a certain position at the initial stage, the size of the voltage applied to the motor 13 is regularly increased.
That is, as shown in FIG. 2A, a duty ratio (an ON/OFF ratio of 6 transistor switches of the inverter unit 12) is controlled so as to be proportionate to the size of a voltage corresponding to each phase, thereby increasing the size of the voltage.
Thereafter, when the microcomputer 15 regularly increases the phase voltage applied to the motor 13, the speed of the inner tub directly connected to the motor 13 or the pulsator is gradually increased.
In the case that the speed is increased, as shown in FIG. 2B, if there is much laundry weight compared to the reference laundry weight, the motor 13 should generate more torque, so that more time is taken for the motor 13 or the inner tub to reach the reference speed or the reference position than the reference time.
Thus, during the time when a certain torque is applied to the motor 13, if the laundry weight is more than the reference laundry weight, the motor 13 or the inner tub reaches the reference speed or the reference position after the reference time elapse.
Conversely, if the laundry weight is small, the motor 13 or the inner tub reaches the reference speed or the reference position in a shorter time than the reference time.
After a certain voltage is applied to the motor 13, the time during which the motor 13 or the inner tub reaches a certain speed or a certain position is computed, to thereby detect the laundry weight.
However, in the laundry weight detecting method, there are many factors affecting the accuracy of the detected laundry weight, including a mechanical factor such as a deviation of a bearing housing, a vibration difference of the inner tub, a deviation of a damper, a motor factor such as a resistance of the motor 13 or the inductance, and a control circuit factor such as a voltage detect circuit, a voltage variation or a temperature. Among them, the error in the voltage detection according to a power supply voltage variation is the factor that has the biggest influence on detecting of the laundry weight in a washing machine.
As shown in FIGS. 3A through 3D, if power is much used around the washing machine, the size of the power supply voltage inputted to the washing machine is reduced, and a DC link terminal voltage of the inverter unit 12 is varied depending on a limited capacity of a condenser according to the laundry weight and the operation speed in driving the washing machine.
Thus, it is difficult to detect the accurate number of rotation of the motor 13 at the initial stage. In addition, as shown in FIG. 3B, in detecting the laundry weight, if the power supply voltage is increased higher than the reference voltage, since the motor speed reaches the reference speed fast as much as Δt, than Δt0, a laundry detection error occurs corresponding to the time Δt1.
Conversely, if the power supply voltage is reduced lower than the reference voltage, since it reaches the reference speed as much as Δt2, it is detected that there is more laundry corresponding to the time Δt2.
Therefore, the method for detecting the laundry weight with the time during which the motor speed reaches the reference speed or the reference position has the following problem.
That is, in order to accurately compute the laundry weight, a certain voltage should be applied regardless of the laundry weight or the surroundings, but if a certain voltage fails to be applied to the motor due to the surroundings, a computed laundry weight may differ from an actual value.